This invention relates to semiconductor integrated circuit (IC) devices, and particularly to memory arrays.
Solid state IC memory devices are typically formed by a two-dimensional array of cells. Each cell is a transistor that has at least a source, a drain and a control gate. The conduction of the current from the drain to the source depends on the voltage applied to the control gate. A "0" state or a "1" state is possible depending on whether the transistor is conducting or nonconducting. In this way, the transistor functions as a binary memory device.
In electrically programmable read only memory (EPROM, each of the transistors also has a floating gate for charge storage. Electrically erasable programmable read only memory (EEPROM) and Flash EEPROM devices may also have an erase gate in addition to the control gate. During programming, voltages applied to the control gate and drain enable hot electrons to be injected into the floating gate. During erasing, application of voltage at the erase gate enables electrons to be removed from the floating gate. At any time the balance of charge trapped in the floating gate determines the conduction threshold level of the transistor. In this way two or more conduction states are programmable or erasable at each cell. Since the programmed charge tends to be retained at the floating gate, EPROM, EEPROM and Flash EEPROM devices are "non-volatile" memories.
In a two-dimensional array the transistors forming the cells are arranged in rows and columns. An X-Y addressing system is effected by a series of word lines parallel to the rows and a series of bit lines parallel to the columns. A word line connects the gates of all transistors in one row. A bit line connects to either the sources or drains of all transistor in one column.
Two common bit-line arrangements are employed. In one type of arrangement the sources of all the transistors in the array are grounded. In each column, only the drains of all the transistors are tied to a bit line. In another type of arrangement known as a "virtual ground" system, the transistors along each row are daisy-chained together by their sources and drains. In each column of transistors, there are two bit lines. One of the bit lines is a source line that ties all the sources together. The other bit line is a drain line that ties all the drains together. The source line and drain line of a column are respectively also the drain line of the column to the left and the source line of the column to the right. In this way the need for a dedicated source line per column is eliminated. Any memory cell in the array can be programmed or read by application of appropriate voltages to the word line and the bit lines connected to it. In particular, the state of an addressed memory cell can be determined by sensing the current flowing through its source and drain by means of the bit lines connected to them. To reduce the area taken up by the source and drain bit lines they can be implemented as buried diffusion bit lines.
As larger and denser memory arrays are fabricated, several design considerations become more acute. The first problem is that series resistance of the source or drain buried diffusion bit line increases with the number of cells in a column. The buried diffusion bit lines are usually doped silicon diffusions and typically have a resistance of about 50.OMEGA. per cell. For a column with 1024 cells, this can amount to a differential of 50 K.OMEGA. resistance along a bit line.
A similar problem exists with the capacitance of the bit line which also increases with the number of cells in the column. The increase in resistance and capacitance results in a larger RC time constant for the sensing circuit and therefore a slower read cycle. Furthermore, the increase in bit line capacitance results in slower precharge (in cases where the design may require columns to be precharged before read) and higher power spike to charge/discharge bit lines during read.
One solution to reducing bit line series resistance is to parallel the buried diffusion bit lines with lower resistivity metal lines. Typically, aluminum strips are overlaid on the bit lines but generally insulated from them by a dielectric layer in between. A series of metal contact (vias) at intervals along a buried diffusion bit line establish contacts with a corresponding metal strip. The intervals may be as short as 1 contact every 2 cells in high speed static RAM's, dynamic RAM's, ROM, EPROM, EEPROM or flash EEPROM.
While paralleling with metal lines assists in reducing bit-line resistance, metallization and contacts introduce problems of their own. In high density integration, there is increasing danger of shorts between metal strips due to their narrow spacing and imperfect contacts which can adversely affect performance. The current state of technology limits the pitch (line width plus spacing) of the metal lines to be about 1.5 .mu.m.
It is known that one way of controlling bit line series resistance and capacitance is to break up the columns and therefore the bit lines into shorter segments. One example is described in a conference paper entitled "A 16 Mb DRAM With An Open Bit-Line Architecture", by M. Inoue et al, published in ISSCC, Feb. 19, 1988. The paper demonstrates a 16 Mb DRAM array segmented into segments of 256 word lines each. A dedicated sense amplifier is connected to each segmented bit line at one end and to a global bit line at the other end via a select transistor. The authors show that in conventional open bit line architectures, the total packing density of a 16 MB DRAM cannot be improved since the scaling of the cell size is limited by the layout pitch of the sense amplifiers. To overcome this problem, the authors arrange the sense amplifiers for adjacent bit lines alternately at the top and bottom of the segment. This allows each sense amplifier to occupy double the spacing of the columns. However, the architecture has the disadvantage of using multiple sense amplifiers; one set is used for every segment. The pitches of the metal lines and the select transistors remain the same as that of the bit lines.
In another conference paper entitled, "16 Mb ROM Design using Bank Select Architecture", by M. Okada et al, published in IEEE Symposium on VLSI Circuits, Tokyo, August 1988 p. 85, a segmented array is also employed to reduce bit line series resistance. The ROM is divided into 256 banks. Each bank is essentially a segmented column consisting of 16 word lines. Only one set of sense amplifiers is used for the whole array, and it is selectively switched onto even or odd columns by means of aluminum lines. The pair of segmented bit lines in each column are connectable to a pair of aluminum lines by bank-select transistors at all four ends, at the top and bottom of the segment. The aluminum lines run zigzag in the column direction between even and odd adjacent columns, and alternate in the middle of each segment. This architecture is such that the pitch of the aluminum lines is double that of the bit lines. This allows the aluminum lines to be further apart, thereby reducing the possibility of shorts. However, the disadvantage is that the pitch of the select transistors remains the same as that of the bit line and therefore limits the ultimate size and therefore the conductance of the select transistors. In addition, the bit line capacitance is not optimally reduced because the bank selection is such that when a column is selected, its source line and drain line are respectively shorted with the source line of the left adjacent column and the drain line of the right adjacent column. The selected bit lines are therefore coupled to the capacitance of other bit lines that are not selected.
The aforementioned architectures while suitable for mask programmed ROM or DRAM are not applicable to programmable memories such as EPROM or Flash EEPROM devices. These devices are electrically programmable and for programming require a much higher drain-source current (about one or two orders of magnitude higher) than that during reading. In a segmented architecture, the select transistors used must be capable of selectively applying the higher programming current involved. To do so the size of the select transistors must be large enough to limit their internal resistance. Prior art architectures restrict each select transistor to fit within the pitch of a column, which limits the size of the select transistors without unduly increasing the separation between adjacent segments.